The objectives of the proposed research are to delineate the intrathymic events in the acquisition of T cell immunocompetence by stem cells. We propose to focus on the questions of what steps are involved in "thymus derivation" of lymphocytes, what changes occur in surface topography in the course of a stem cell traversing the thymus, what recognition phenomena are involved that commit a precursor cell to thymus derivation, to establish by histological and ultrastructural means whether there is direct cell-to-cell interaction between the thymus epithelium and stem cells and if this occurs by means of transient gap junctions. To do this we have established a system of whole organ thymus culture which mimics in vivo the in vitro physiological situation of the gland by taking advantage of its blood supply using a microperfusion system. Such a perfused gland can confer immunocompetence on passaged stem cells. The latter are recovered from bone marrow by centrifugation on discontinuous albumin gradients or enriched by column purification. We propose to examine the surface membrane characteristics before and after perfusion using antibodies to known determinants such as H2, theta, Ig, Fc, and C3, and to modulate these membranes with ligands and antibody.